1. Field of the Invention
The present invention relates to a contact structure in an electronic device such as but not limited to a mobile communication terminal.
2. Background of the Related Art
Most mobile communication terminals have keypads for inputting characters or symbols, and the keypads usually have a contact sheet. In a related art terminal, the sheet is attached to a surface of a PCB (printed circuit board) having a plurality of keypad switches ringed thereon. The sheet includes a plurality of metal domes made of a metal based material (e.g., stainless steel) and the domes are attached onto the keypad switches respectively.
In operation, when a user presses a key (e.g., a character key), pressure is placed on the metal dome beneath the pressed key. This pressure causes the key to move and thus become distorted in a downward direction. As a result, the corresponding keypad switch is pressed by such distortion to bring about switch operation. The metal dome is useful because it gives a user's finger a feeling of click when pressed by the user, and also protects the corresponding keypad switch beneath the metal dome.
A related art keypad includes a plurality of keys (character keys) formed by injection molding of synthesized resin such as acryl or the like. The keys have various characters or symbols formed thereon so that they may be distinguished from each other. Over time, a character or symbol printed on a surface of the key wears out and thus is hardly recognizable. To overcome this drawback, a specific color is coated on an inner face of the key using pigments. This color is partly removed to display a character or symbol. This method is widely used.
More specifically, in the foregoing keypad the character (symbol) is depicted by stripping off a portion of the colored surface using a laser. The portion representing the character is the stripped portion from which the color is removed, and it is transparent to allow for leakage of light emitted from inside. Hence, the user is able to distinguish and select keys conveniently. Also, it is noted that keypad switches are placed beneath the keypad to generate electric signals corresponding to the pressed keys. Once the key is pressed, the keypad switch is operated to generate the corresponding electric signal.
As previously discussed, a metal dome is frequently placed between the key and keypad switch to provide a “click” feeling to a user as well as to protect the switch. The keypad is formed from a plurality of the keys and an insulating sheet is usually attached to support a plurality of metal domes. An insulating sheet having metal domes attached thereto may be referred to as a metal dome sheet
FIGS. 1 and 2 show different views of a metal dome sheet according to the related art. The metal dome sheet 10 includes a thin plate type insulating sheet 15 and metal domes 1 attached to the insulating sheet. Each metal dome 1 is made of a substantially hemispherical stainless steel plate. More specifically, each metal dome formed like a contact lens by cutting a portion of a sphere.
The insulating sheet 15 is mainly made of a white plastic sheet, and the number of the metal domes coincides with the number of keys to be placed on the metal domes. A plurality of holes 2 are formed between the metal domes. Light emitted from LEDs installed on a PCB (printed circuit board) having keypad switches, passes through the holes.
Problems may arise as a result of using the metal dome sheet of the related art when used in communications terminals. In these terminals, the keys (keyboard) have a tendency to be too small to use. Hence, key size has to be increased.
As the size of the keys (keyboard) increases, so does a display portion for each number or character as well as width of each character style. However, if the size of the key or the width of the character or symbol increases, the corresponding metal dome beneath the key is seen externally through the wider and larger character potion of the key (keyboard).
A cut portion (numeral and character portion) of the key, which is cut by a laser, becomes dark due to the color of the corresponding metal dome. As a result, the light leaking through the key becomes hazy. Moreover, the metal dome protrudes out of the surface of the sheet to form a dome shape and blocks the light-propagating space to inhibit the flow of light. In FIG. 1, a light path is partially shown by a cutting line to show that light emitted from LED is blocked by the metal domes. Consequently, use of the metal domes inhibits bright illumination of the keys of the keyboard. Moreover, the metal domes are basically made of an opaque material, which thereby worsens the illumination problem.
A need therefore exists for an improved contact sheet which enables light to be evenly applied to a keypad, and which also prevents the color of the contacts from being exposed naturally.